Geophysical investigation of fens in south-central Oregon: implications for stratigraphic controls on groundwater dependence

Abstract:

The formation of montane fens is dependent on the presence of groundwater discharge at a rate that supports the growth and accretion of peat, a process that is reliant upon the interrelationship of geologic structure and hydrology and biologic controls. The fen plays an integral role in the freshwater landscape as a headwater or as point discharges to subsurface hydrostratigraphic layers (Aldous et al 2015). South-central Oregon is underlain by a series of volcanic, pumice-laden deposits that create a unique series of subsurface hydrostratigraphic layers dispersed across local and regional scales that show a spatial complexity difficult to map with traditional coring methods that may also cause disruption of the soil matrix.

Geophysical methods have proved the ability to non-invasively image the complex architecture of the critical zone for decades. This research used a combination of ground penetrating radar (GPR), and terrain conductivity constrained with hydrological and coring measurements and vegetation surveys to image the spatial distribution of pumice layers across several fens in the Deschutes and Klamath Basins in Oregon, and infer how subsurface stratigraphy may influence the surface characteristics of these fens. Datasets were collected using an array of techniques to infer subsurface properties at different scales of measurement, from 1D models at discrete locations to estimate water content distribution, to 3D models to demonstrate the ability of near-surface geophysics to characterize the underlying geology of fens in a volcanic deposit rich environment. Understanding how the surface and subsurface structure contributes to these unique hydrogeologic settings could bolster conservation and management plans in response to groundwater pumping and climate change, as well as understand the fundamental mechanisms for the preferential flow of water through the unconsolidated volcanic materials.